In liquid crystal display (LCD) devices, the interior of the windows are coated with mechanically rubbed polymer layers to align the liquid crystal (LC) medium into a single domain. (Chigrinov, Liquid Crystal Devices: Physics and Applications (Artech House, Boston, 1999).) This practice has several disadvantages: The rubbing cloth is a source of contamination and electrostatic charging. The rubbed polymer is very difficult to pattern. And, it is impossible to control the alignment after cell assembly. To overcome these limitations, Ichimura and Gibbons initiated studies of organic thin films containing photosensitive groups for non-contact alignment. (Ichimura et al., Langmuir 4:1214 (1988); Gibbons et al., Nature 351:49 (1991).) Numerous variations have been reported, as has been documented in a recent review. (Ichimura, Chem. Rev. 100:1847 (2000).) Among the most successful designs are those that exploit the trans-cis-trans photoisomerization cycle of azobenzene. The absorption cross section for this process depends on the direction of the optical electric field. Therefore, an ensemble of azobenzene groups will develop photoinduced anisotropy (PIA) under polarized illumination as the molecules are agitated into low absorption orientations. (Sekkat and Knoll, Photoreactive Organic Thin Films (Academic Press, San Diego, 2002).) The azobenzene structure is similar to the rigid unit of common LC molecules, which promotes effective communication of the anisotropy to an adjacent LC medium. Therefore, the orientation of the LC next to this surface can be externally controlled. In a useful device, the opposite surface must maintain fixed alignment through strong anchoring. Typical embodiments are hybrids that involve the light-controlled layer on one window and a conventionally rubbed polymer on the opposite window. (Ichimura et al., Langmuir 4:1214 (1988); Gibbons et al., Nature 351:49 (1991); Ichimura et al., Liquid Crystals 20:423 (1996); Schadt et al., Nature 381:212 (1996); Palffy-Muhoray et al., Appl. Phys. A, 75:293 (2002).) While this configuration enables photoinduced switching after assembly of the cell, the rubbed polymer compromises the advantages of non-contact alignment.
Accordingly, photoinduced anisotropy has been used in hybrid liquid crystal light valves in the past. However, these hybrid cells contain one surface that is aligned by contact rubbing methods, which introduce contamination, scratches, and electrostatic charging, all of which limit the quality that can be achieved in a device.